1. Field of the Invention
The present invention relates to a motor-operated power steering apparatus, and more particularly to a motor-operated power steering apparatus in which the heating of an electric motor and a driver therefor is effectively suppressed.
2. Description of the Relevant Art
There are known motor-operated power steering apparatus in which the power of an electric motor is transmitted to a steering force transmitting mechanism for assisting the steering force. In such a motor-operated power steering apparatus, a relatively large current flows through the motor to cause the motor and its driver circuit to be heated. These components tend to be also heated to high temperature by the heat from other heated parts. Japanese Laid-Open Utility Model Publication No. 61-91465 discloses a motor-operated power steering apparatus in which the components are protected from a high-temperature environment. In the disclosed motor-operated power steering apparatus, when the temperature of the motor exceeds a predetermined temperature, the current flowing through the motor is limited to suppress the heating of the motor and other heated components.
The current flowing through the motor is limited on the basis of the temperature of the motor which is of relatively large heat capacity. Therefore, heat control may not properly reflect the thermal condition of the motor and other parts. For example, even if the current supplied to the motor is not to be limited because the temperature of the motor has started to be lowered, the current is limited when the motor temperature remains higher than the predetermined temperature. As a result, the current fed to the motor is limited for a long period of time, thus impairing the driver's feeling as to the steering action.